rcu: Restrict access to RCU CPU stall notifiers

Although the RCU CPU stall notifiers can be useful for dumping state when
tracking down delicate forward-progress bugs where NUMA effects cause
cache lines to be delivered to a given CPU regularly, but always in a
state that prevents that CPU from making forward progress. These bugs can
be detected by the RCU CPU stall-warning mechanism, but in some cases,
the stall-warnings printk()s disrupt the forward-progress bug before
any useful state can be obtained.

Unfortunately, the notifier mechanism added by commit 5b404fdabacf ("rcu:
Add RCU CPU stall notifier") can make matters worse if used at all
carelessly. For example, if the stall warning was caused by a lock not
being released, then any attempt to acquire that lock in the notifier
will hang. This will prevent not only the notifier from producing any
useful output, but it will also prevent the stall-warning message from
ever appearing.

This commit therefore hides this new RCU CPU stall notifier
mechanism under a new RCU_CPU_STALL_NOTIFIER Kconfig option that
depends on both DEBUG_KERNEL and RCU_EXPERT. In addition, the
rcupdate.rcu_cpu_stall_notifiers=1 kernel boot parameter must also
be specified. The RCU_CPU_STALL_NOTIFIER Kconfig option's help text
contains a warning and explains the dangers of careless use, recommending
lockless notifier code. In addition, a WARN() is triggered each time
that an attempt is made to register a stall-warning notifier in kernels
built with CONFIG_RCU_CPU_STALL_NOTIFIER=y.

This combination of measures will keep use of this mechanism confined to
debug kernels and away from routine deployments.

[ paulmck: Apply Dan Carpenter feedback. ]

Fixes: 5b404fdabacf ("rcu: Add RCU CPU stall notifier")
Reported-by: Linus Torvalds <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>
Reviewed-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Neeraj Upadhyay (AMD) <[e-mail]>

Documentation/admin-guide/kernel-parameters.txt

···
 			Dump ftrace buffer after reporting RCU CPU
 			stall warning.
 
+	rcupdate.rcu_cpu_stall_notifiers= [KNL]
+			Provide RCU CPU stall notifiers, but see the
+			warnings in the RCU_CPU_STALL_NOTIFIER Kconfig
+			option's help text.  TL;DR:  You almost certainly
+			do not want rcupdate.rcu_cpu_stall_notifiers.
+
 	rcupdate.rcu_cpu_stall_suppress= [KNL]
 			Suppress RCU CPU stall warning messages.
 

include/linux/rcu_notifier.h

···
 #define RCU_STALL_NOTIFY_NORM	1
 #define RCU_STALL_NOTIFY_EXP	2
 
-#ifdef CONFIG_RCU_STALL_COMMON
+#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #include <linux/notifier.h>
 #include <linux/types.h>
···
 int rcu_stall_chain_notifier_register(struct notifier_block *n);
 int rcu_stall_chain_notifier_unregister(struct notifier_block *n);
 
-#else // #ifdef CONFIG_RCU_STALL_COMMON
+#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 // No RCU CPU stall warnings in Tiny RCU.
 static inline int rcu_stall_chain_notifier_register(struct notifier_block *n) { return -EEXIST; }
 static inline int rcu_stall_chain_notifier_unregister(struct notifier_block *n) { return -ENOENT; }
 
-#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #endif /* __LINUX_RCU_NOTIFIER_H */

kernel/rcu/Kconfig.debug

···
 	  The boot option rcupdate.rcu_cpu_stall_cputime has the same function
 	  as this one, but will override this if it exists.
 
+config RCU_CPU_STALL_NOTIFIER
+	bool "Provide RCU CPU-stall notifiers"
+	depends on RCU_STALL_COMMON
+	depends on DEBUG_KERNEL
+	depends on RCU_EXPERT
+	default n
+	help
+	  WARNING:  You almost certainly do not want this!!!
+
+	  Enable RCU CPU-stall notifiers, which are invoked just before
+	  printing the RCU CPU stall warning.  As such, bugs in notifier
+	  callbacks can prevent stall warnings from being printed.
+	  And the whole reason that a stall warning is being printed is
+	  that something is hung up somewhere.	Therefore, the notifier
+	  callbacks must be written extremely carefully, preferably
+	  containing only lockless code.  After all, it is quite possible
+	  that the whole reason that the RCU CPU stall is happening in
+	  the first place is that someone forgot to release whatever lock
+	  that you are thinking of acquiring.  In which case, having your
+	  notifier callback acquire that lock will hang, preventing the
+	  RCU CPU stall warning from appearing.
+
+	  Say Y here if you want RCU CPU stall notifiers (you don't want them)
+	  Say N if you are unsure.
+
 config RCU_TRACE
 	bool "Enable tracing for RCU"
 	depends on DEBUG_KERNEL

kernel/rcu/rcu.h

···
 	return rcu_cpu_stall_suppress_at_boot && !rcu_inkernel_boot_has_ended();
 }
 
+extern int rcu_cpu_stall_notifiers;
+
 #ifdef CONFIG_RCU_STALL_COMMON
 
 extern int rcu_cpu_stall_ftrace_dump;
···
 bool rcu_cpu_beenfullyonline(int cpu);
 #endif
 
-#ifdef CONFIG_RCU_STALL_COMMON
+#if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 int rcu_stall_notifier_call_chain(unsigned long val, void *v);
-#else // #ifdef CONFIG_RCU_STALL_COMMON
+#else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 static inline int rcu_stall_notifier_call_chain(unsigned long val, void *v) { return NOTIFY_DONE; }
-#endif // #else // #ifdef CONFIG_RCU_STALL_COMMON
+#endif // #else // #if defined(CONFIG_RCU_STALL_COMMON) && defined(CONFIG_RCU_CPU_STALL_NOTIFIER)
 
 #endif /* __LINUX_RCU_H */

kernel/rcu/rcutorture.c

···
 	unsigned long stop_at;
 
 	VERBOSE_TOROUT_STRING("rcu_torture_stall task started");
-	ret = rcu_stall_chain_notifier_register(&rcu_torture_stall_block);
-	if (ret)
-		pr_info("%s: rcu_stall_chain_notifier_register() returned %d, %sexpected.\n",
-			__func__, ret, !IS_ENABLED(CONFIG_RCU_STALL_COMMON) ? "un" : "");
+	if (rcu_cpu_stall_notifiers) {
+		ret = rcu_stall_chain_notifier_register(&rcu_torture_stall_block);
+		if (ret)
+			pr_info("%s: rcu_stall_chain_notifier_register() returned %d, %sexpected.\n",
+				__func__, ret, !IS_ENABLED(CONFIG_RCU_STALL_COMMON) ? "un" : "");
+	}
 	if (stall_cpu_holdoff > 0) {
 		VERBOSE_TOROUT_STRING("rcu_torture_stall begin holdoff");
 		schedule_timeout_interruptible(stall_cpu_holdoff * HZ);
···
 		cur_ops->readunlock(idx);
 	}
 	pr_alert("%s end.\n", __func__);
-	if (!ret) {
+	if (rcu_cpu_stall_notifiers && !ret) {
 		ret = rcu_stall_chain_notifier_unregister(&rcu_torture_stall_block);
 		if (ret)
 			pr_info("%s: rcu_stall_chain_notifier_unregister() returned %d.\n", __func__, ret);

kernel/rcu/tree_stall.h

···
 }
 early_initcall(rcu_sysrq_init);
 
+#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
 
 //////////////////////////////////////////////////////////////////////////////
 //
···
  */
 int rcu_stall_chain_notifier_register(struct notifier_block *n)
 {
-	return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
+	int rcsn = rcu_cpu_stall_notifiers;
+
+	WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
+	     rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
+	if (rcsn)
+		return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
+	return -EEXIST;
 }
 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
 
···
 {
 	return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
 }
+
+#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER

kernel/rcu/update.c

···
 EXPORT_SYMBOL_GPL(torture_sched_setaffinity);
 #endif
 
+int rcu_cpu_stall_notifiers __read_mostly; // !0 = provide stall notifiers (rarely useful)
+EXPORT_SYMBOL_GPL(rcu_cpu_stall_notifiers);
+
 #ifdef CONFIG_RCU_STALL_COMMON
 int rcu_cpu_stall_ftrace_dump __read_mostly;
 module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
+#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
+module_param(rcu_cpu_stall_notifiers, int, 0444);
+#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
 int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings.
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
 module_param(rcu_cpu_stall_suppress, int, 0644);